    [Patent Document 1] JP 2000-348300 A
A technology of convoy is known in which a trailing vehicle trails a leading vehicle at a predetermined inter-vehicle distance by use of information transmitted from the leading vehicle via inter-vehicle communications. For example, Patent Document 1 discloses a technology that, since each vehicle other than the last vehicle may be a leading vehicle during convoy, each vehicle periodically transmits trailing information about its own position, operational amounts (steering, acceleration, and brake), and momentums (speed, acceleration, and yaw rate) to its trailing vehicle, and each trailing vehicle trails its leading vehicle on the basis of the received trailing information.
However, congestion is likely to occur in the technology of Patent Document 1. In the technology of Patent Document 1, since trailing information is periodically transmitted from each vehicle through inter-vehicle communications, trailing information is likely to occupy a communication band when many vehicles are present in the convoy, and thus congestion in communications is likely to occur.